1. Field of the Invention
The present invention relates to three-phase AC induction motors. More particularly, the present invention relates to three-phase AC induction motors which utilize squirrel cage rotors. More particularly, the present invention relates to squirrel cage motors which utilize dual stators. The present invention also relates to drilling apparatus that have motors for driving a winch drum associated with paying in and out a drilling string.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 37 CFR 1.98
The use of electrical motors is well known. The most commonly used electrical motor is the three-phase AC induction motor. In induction motors, the rotor assembly comprises a generally cylindrically-shaped unit wherein the sides of the unit are formed by a series of spaced electrically conductive metal bars, extending from one end of the rotor to the other. An associated stator assembly is disposed around the rotor and includes electrical windings. When electrical current is applied to the stator windings, the electrical field provided creates eddy currents in the metal bars, causing the bars to move, and thereby, the rotor to rotate within the stator. Because of the cage-like configuration of the rotor, it is commonly referred to as a squirrel cage rotor, and motors in which such rotors are present are referred to as “squirrel cage” induction motors.
While there are other types of rotors, most AC motors use the squirrel cage rotor. These rotors are found in almost all domestic and light industrial AC motors. For example, CD and DVD players may use very small squirrel cage motors, producing about twenty-five watts of power, and appliances such as washing machines may use squirrel cage motors producing about 250 watts of power. These motors are capable of producing much more power, as motors for use by pipeline compressors may produce tens of thousands of kW of output.
Large AC motors are often used on offshore oil rigs, serving to power the drawworks or other systems. The drawworks is a machine that is used to raise and lower a line connected to a drill string. Conventional drawworks were once DC motor-driven systems with transmissions typically geared through the use of chain and sprocket drive systems. These drawworks used pneumatic clutches and frictions to mechanically tie the selected gearing to the drive shaft eventually coupling the DC machines to the drawworks via a speed and torque ratio. Using this configuration, the DC motors were uncoupled during the lowering functions and a system of brakes were used to lower the drill string.
More recently, oil rigs have used gear-driven drawworks with fixed ratios using standard AC induction motors. When using fixed gearbox ratios, the amount of torque required in lifting heavy loads dictates a ratio that minimizes the braking torque during lowering. In AC motor applications, once the nominal speed of the machine is greater than its design rating, the torque produced for a given ampere is inversely proportional to the increase of speed over the nominal rated speed. Once again, this dictates the need for braking means above and beyond the braking torque capacity of the electric motor.
Another issue encountered when using gear driven drawworks is the inertia of the rotors when driven by electric motors. It has been found that a large part of the braking power required in slowing down or speeding up the drawworks is dictated by the system inertia. Drawworks must often compensate for the heave encountered when working on floating rigs. This makes the effect of the inertia very important and a high inertia leads to greater fuel consumption, greater power requirements, and increased emissions.
In applications other than offshore drilling, some drawworks utilize permanent magnet technology. Problems in the cost of rare earth magnets, temperature considerations and magnetic saturation makes this an expensive option and not as reliable as induction technology.
The present invention may be used as part of a “top drive” direct drive system associated with a drilling rig. In a top drive system, a motor or series of motors is suspended from a derrick. The motor's shaft is coupled to a short section of pipe or pipe-handling apparatus that is connected to the drill string. The action of the motor directly rotates the drill string without the use of a gearbox. Since the motor and pipe handling apparatus are suspended well above the rig floor, the system can handle multiple lengths of pipe at once. As the well is drilled, the motor and pipe-handling apparatus are lowered and then raised again when there is a need to affix more pipe to the drill string. The motor of the present invention is suitable for use in a top drive system particularly because of its lower torque to weight ratio and its smaller size.
Many patents have issued that are relevant to the current invention. For example, U.S. Pat. No. 5,783,893, issued on Jul. 21, 1998, teaches an electric machine, such as a motor or generator, configured in a machine housing with a cup-shaped rotor attached to a shaft rotatively supported in the housing, an inner stator coaxial with and enclosed by the rotor, and an outer stator coaxial with and enclosing the rotor. The inner and outer portions of the machine are separated by a magnetic isolator that divides and supports inner and outer portions of the rotor. The magnetic isolator, which supports inner and outer sets of permanent magnets in one embodiment and inner and outer squirrel cage rotor assemblies in another embodiment is preferably made of non-magnetic stainless steel but may also be made of titanium, brass, aluminum, bronze or magnesium, or any material that provides magnetic isolation between the inner and outer portions of the machine.
U.S. Pat. No. 5,525,851, issued on Jun. 11, 1996 to Kumamoto et al., describes an apparatus for producing high-speed rotation which has a rotary shaft integrally having a first rotor with magnetic anisotropy, and a rotary sleeve disposed around the rotary shaft. The rotary sleeve has a first stator surrounding the first rotor and integrally has a second rotor having magnetic anisotropy. The first stator has a field winding for magnetizing the first rotor in a predetermined direction. This winding is also an armature winding because it undergoes high-speed rotation with the second rotor. The rotary sleeve is received in a housing which has a second stator surrounding the second rotor. The second stator has a field winding for magnetizing the second rotor in a predetermined direction and for producing high-speed rotation of the second rotor. By analogy, the housing can become a further rotary sleeve, constructed similarly to the above-mentioned rotary sleeve.
U.S. Pat. No. 6,819,026, issued on Nov. 16, 2004 to Narita et al., teaches a radial-air-gap induction motor constituted so as to have two air gaps by setting a rotor between a cylindrical outer stator and inner stator in order to improve an efficiency by increasing the ratio of torque/(square of current) by a two air gaps, apply windings for generating a rotating magnetic field to the outer and inner stators, and form squirrel-cage windings on the rotor. This patent claims priority of U.S. Patent Publication No. 2003/0201686, published on Oct. 30, 2003.
U.S. Patent Publication No. 2003/0155832, published on Aug. 21, 2003 by Herren, teaches an electric motor additive speed drive assembly which comprises at least two incrementally increasing diameter electric motors mounted in an integrated concentric manner along a common axis such that the speed and horsepower of the assembly output shaft is the sum of the speed and horsepower of the energized electric motors in the assembly. At least one set of electrically conductive end bearings spans the annular space between the casings of each motor in the assembly allowing the stators of each internal motor to rotate. Electricity is conducted to each inner motor through the electrically conductive end bearings.
U.S. Pat. No. 6,031,312, issued on Feb. 29, 2000 to Zoche et al., describes a squirrel cage rotor that includes a cage, lamination sheets provided with a plurality of slots, and short circuit rings arranged at the axial ends of the lamination sheets and connected by short circuit rods disposed within the slots. In one aspect, the lamination sheets comprise sheets made of an amorphous magnetic material, especially of an amorphous metal with high saturation magnetization. In another aspect, the slots are formed with a convex base portion. In a further aspect, the short circuit rings are made of a metal matrix composite material with high electrical conductivity, preferably of a fiber-reinforced aluminum matrix composite material.
U.S. Pat. No. 4,064,410, issued on Dec. 20, 1977 to Roach, teaches a rotor for use in a dynamo-electric machine comprising a shaft, a laminated magnetic core carried by the shaft, a plurality of arcuately spaced-apart rotor bars carried by the core and having end portions protruding beyond the end laminations at opposite ends of the core, and a pair of end rings disposed at opposite ends of the core and joined to the protruding end portions of the rotor bars. The axially facing inner end surface of each end ring has a series of arcuately-spaced radially-extending channels formed therein with intervening arcuately-spaced radially-extending ribs defined therebetween, with the channels receiving the protruding end portions of the rotor bars therein, and with the ribs bearing tightly against the end laminations at opposite ends of the laminated core to maintain the laminated core in tightly compressed condition.
U.S. Pat. No. 5,444,319, issued on Aug. 22, 1995 to Nakamura et al., teaches a squirrel-cage rotor which includes a laminated core fixed to a rotor shaft, a plurality of secondary conductors arranged respectively in a plurality of through holes formed through the laminated core, a pair of end rings connected to the secondary conductors at axial ends of the laminated core, and a pair of reinforcing members covering the end rings. The secondary conductors and the end rings are integrally formed through a casting process, and are connected with the laminated core and the reinforcing members. Each reinforcing member is formed as an annular element of high-rigidity material, and includes a cylindrical wall surrounding the cylindrical outer surface of the end ring, a multi-aperture wall provided with a plurality of apertures which communicate respectively with the through holes of the laminated core and held between the end ring and the laminated core, and an end wall brought into contact with the axial outer surface of the end ring.
U.S. Pat. No. 7,030,528, issued on Apr. 18, 2006 to Morgante, teaches a dual concentric AC motor which allows for two independently operating AC motors that produce the same torque at the same current input as two conventional, separate electric motors while occupying a smaller physical volume. The dual concentric AC motor utilizes a single, hollow cylindrical stator core comprising inner and outer stators and an inner rotor and an outer rotor that operate independently of one another. The inner stator, with windings that face toward the center of the motor, couples to the inner rotor, which rotates inside the single stator core, while the outer stator, with windings that face away from the center of the motor, couples to the outer rotor, which rotates around the single stator core. A back iron, centrally located in the single stator core, physically and magnetically separates the inner and outer stators. The two rotors are coupled to separate, independent output shafts.
U.S. Pat. No. 5,281,879, issued on Jan. 25, 1994 to Satake et al., teaches a synchronous motor which includes a unitary rotor, a first stator, a second stator and phase-changing switches. The unitary rotor has a first rotor portion formed by a first permanent magnet and a second rotor portion formed by a second permanent magnet and an induction type rotor. These first and second rotor portions are mounted on a common rotary axle with a predetermined space provided therebetween. The first stator faces the first rotor portion for producing a first rotating magnetic field. The second stator faces the second rotor portion for producing a second rotating magnetic field and is disposed such that, at the starting operation, the attracting action or the repelling action produced between the first rotating magnetic field and the first permanent magnet is canceled by the repelling action or the attracting action produced between the second rotating magnetic field and the permanent magnetic field. The phase-changing switches are associated with either one of the first stator or the second stator and set a phase difference of 0 degree or 180 degrees between the first rotating magnetic field and the second rotating magnetic field. Due to the cancellation action, any starting interference which may otherwise be caused by the permanent magnets is made negligible.
U.S. Pat. No. 4,829,205, issued on May 9, 1989 to Lindgren, describes an induction-motor structure having a synchronous-rotor and an induction-rotor in which inner and outer synchronous-rotor poles and a synchronous-rotor-pole connector surround the inner-cylindrical surface of the hollow cylindrical induction rotor. The outer-cylindrical surface of a hollow cylindrical stator core has alternating-current windings and surround the ends of that induction rotor and that core. At least one of the rotor poles includes magnetic field concentrators. A stationary field winding is mounted on the end of the stator and may be used to adjust the power factor.
U.S. Pat. No. 4,920,293, issued on Apr. 24, 1990 to Kanda, teaches a squirrel-cage induction motor having a sectional stator comprising a first stator fixedly held on a frame, and a second stator circumferentially movably supported on the frame beside the first stator. The second stator is displaced relative to the first stator through a gear mechanism by a control motor through an angle meeting operating conditions of the squirrel-cage motor required by the load. Thus, the squirrel-cage induction motor has satisfactory response characteristics, and the second stator can manually be turned in case the component of the mechanism for driving the second stator (for example, the control motor) malfunctions.
It is an object of the present invention to provide a squirrel cage induction motor which optimizes intermittent power density.
It is a further object of the present invention to provide a squirrel cage induction motor which optimizes peak torque capability.
It is a further object of the present invention to provide a squirrel cage induction motor which minimizes motor weight.
It is a further object of the present invention to provide a squirrel cage induction motor which increases torque-to-weight ratio.
It is a further object of the present invention to provide a squirrel cage induction motor which minimizes inertia associated with the motor.
These and other objects and advantages of the present invention will become apparent from a reading of the attached specification and appended claims.